The ability to determinate the location of an emitter based upon the reception of its transmitted electromagnetic energy has many military and commercial applications. Military applications include locating the emitters associated with enemy weapon and communication systems. Commercial applications include locating cell-phones for emergency services (E-911), a requirement mandated by the FCC for wireless carriers in the United States. Other uses include the ability to locate stolen vehicles and other items which transmit distress signals (i.e. similar to LOJAC and ONSTAR). Inventory or items marked by RFID tags can be located, as well. These uses may all be enhanced by minimizing the uncertainty associated with locating the emitter.
Historically, interferometric Direction Finding (DF) techniques have been utilized for emitter location. However, the accuracy obtained from DF techniques is not sufficient in all instances. When multiple, spatially-separated antenna/receiver systems detect a signal transmitted by an emitter, the signal is detected by each of the antenna/receiver systems at a different Time-Of-Arrival (TOA). The range from the emitter to the antenna/receiver system of interest determines the TOA of the signal at each antenna/receiver system. Since the emitted electromagnetic waves propagate at the speed of light, the emitter's location may be inferred based upon the time difference of a signal's arrival at each antenna/receiver system. This requires that the location of each system be known and the receivers share a common time-reference. Emitter location methods based upon different reception times of a signal by spatially separated antenna/receiver systems are referred to as Time-Difference-Of-Arrival (TDOA) techniques. Often, TDOA emitter location techniques offer a vast improvement in accuracy over that provided by DF techniques.
FIG. 1 depicts the TDOA measured between a pair of antenna/receiver systems 12 and 14 that produces a hyperboloid of revolution that describes the possible emitter 10 locations with respect to the positions of the receivers. The intersection of this hyperboloid with the Earth's surface forms a contour 18 of constant TDOA that describes the possible location of the emitter. This contour may be referred to as an “isochron”. Isochrons are not infinitely thin. Each isochron possesses a thickness, or width 20, that is a function of the geometry between the antenna/receiver pairs 12 and 14 with respect to emitter 10. The width is also a function of errors inherent in the measurement of the TOA of the signal at each of the receivers and the error associated with the knowledge of the receivers' positions. A single isochron 18 cannot determine the location of emitter 10. Rather, the intersection of at least two isochrons is required to determine the location of emitter 10. Since the isochrons have an associated thickness 20, the intersection of isochrons produces an uncertainty region that includes the true location of the emitter. Traditionally, the error between the estimated emitter location and the true emitter location is statistically expressed as a Circular Error Probable (CEP) Target Location Error (TLE) value.
As these errors are often uncharacterized and assumed to be equal, a need exists for the ability to properly evaluate and minimize the error or width of any given isochron and the CEP TLE associated with a pair of isochrons.